1. Field of the Invention
The present invention relates generally to a power steering system for vehicles, and more particularly to an electric power steering system for vehicles which produces auxiliary torque for steering by means of a steering servo unit using an electric motor and which has a function of trouble diagnosis.
2. Description of Relevant Art
Recently, in view of the problems of hydraulic type power steering systems such as the structure thereof is complicated, a variety of electric type power steering systems for vehicls have been proposed.
Exemplarily, in UK Patent Application Publication GBA No. 2132950, published July 18, 1984, there is disclosed an electric power steering system for vehicles of an analogue control type.
This power steering system includes a steering servo unit having an electric motor as its power source, and an analogue control circuit for controlling the servo unit in accordance with a steering torque signal from a detection circuit which detects steering torque acting on a steering wheel. The electric motor produces auxiliary steering torque to thereby reduce the necessary force for steering operation.
In recent years, there is a tendency to utilize microcomputers, which fundamentally are for processing digital signals, as control devices of various working systems, since they have the advantage that complicated control functions can be achieved with a relatively simple system constitution. It is thus desirable to employ a micro-computer as a control device for such a steering system as described, while providing a diagnostic function of various troubles to thereby increase the system reliability.
However, in general, the micro-computer is unable to simultaneously read many input signals, and signal process operations thereof are sequentially performed in accordance with a system clock pulse. Therefore, in the case a variety of trouble diagnoses are left to a micro-computer, there is the possibility that the responsiveness in motor drive control to steering operations may become insufficient. Resulted failure in motor drive control to sufficiently follow a steering operation may give an unfavorable steering feeling. If much importance is attached to the responsiveness to steering operations, the trouble-diagnostic function may be degraded, thus lowering the system reliability.
The present invention has been achieved to effectively solve such problems in electric power steering systems for vehicles of the type in which a micro-computer used as a control device has a trouble-diagnostic function.